1. Field of the Invention
This invention relates to a method for separating molecular hydrogen from a mixture of gases comprising the molecular hydrogen. More particularly, this invention relates to a method for separating molecular hydrogen from synthesis gases derived from the gasification of solid hydrocarbon fuels, such as coal and biomass. This invention also relates to the use of dense mixed oxide ion/electronic/hydrogen atom conducting membranes for the hydrogen separation process.
2. Description of Related Art
Solid hydrocarbon fuels such as coal and biomass are converted to gaseous fuels at high temperatures by partial oxidation with air and/or steam. Exemplary of such conversions are processes taught by U.S. Pat. Nos. 4,057,402 and 4,369,045 (coal gasification) and U.S. Pat. Nos. 4,592,762 and 4,699,632 (biomass gasification). Synthesis gases produced by these processes comprise primarily hydrogen and carbon monoxide, typically with a hydrogen/CO molar ratio in the range of about 0.6 to about 6.0. Because of the abundance of solid hydrocarbon fuels, they are potentially major sources of hydrogen, particularly if cost effective means for extracting the hydrogen from the gaseous fuel products can be devised.
Gasification of solid hydrocarbon fuels is carried out at high temperatures in the range of about 600° C. to about 1400° C. Although these temperatures favor the kinetics of chemical reactions, materials selection for use in hydrogen separation is often limited to ceramics. For example, mixed proton/electron conducting ceramics can be used to selectively separate hydrogen from a mixture of gases. The mechanism of hydrogen separation using a mixed proton/electronic ceramic membrane is shown in FIG. 1. As shown therein, a hydrogen-containing gas is introduced on the feed side of the membrane. H2 is dissociated on the membrane surface into protons and electrons. The protons and electrons are transported through the membrane to the opposite surface, the permeate side of the membrane, where they recombine to form H2. The membrane selectivity to H2 is substantially 100%. The flux is expressed as
      J          H      2        =            -              RT                  4          ⁢                      F            2                    ⁢          L                      ⁢                            (                      σ                          H              +                                )                ⁢                  (                      σ            el                    )                            (                              σ                          H              +                                +                      σ            el                          )              ⁢          (                        ln          ⁡                      (                          p                              H                2                            f                        )                          -                  ln          ⁡                      (                          p                              H                2                            p                        )                              )      where R is the gas constant, F is the Faraday constant, L is the membrane thickness, σH+ is the proton conductivity, σel is the electronic conductivity, pH2f is the partial pressure of hydrogen on the feed side of the membrane and PH2p is the partial pressure of hydrogen on the permeate side. The term
                    (                  σ                      H            +                          )            ⁢              (                  σ          el                )                            σ                  H          +                    +              σ        el              =      σ    amb  is called ambipolar conductivity. Thus, the flux is dependent on 1/L, T, σamb, and (ln(pH2f)−ln(pH2p)).
However, mixed proton/electronic conductors have a number of shortcomings including poor strength and reactivity with gasified fuel species such as CO2 and H2S. In addition, H2 separation from the fuel gases renders the spent fuel vulnerable to carbon deposition, which can cover membrane surfaces and inactivate the membrane.
U.S. Pat. No. 5,306,411 teaches solid membranes comprising an intimate, gas impervious, multi-phase mixture of an electronically-conductive material and an oxygen-conductive material and/or a mixed metal oxide of a perovskite structure for use in electrochemical reactors in which oxygen is transported from an oxygen-containing gas to a gas or mixture of gases that consume oxygen, more particularly for partial oxidation of methane to produce unsaturated compounds or synthesis gas, the partial oxidation of ethane, substitution of aromatic compounds, extraction of oxygen from oxygen-containing gases, including oxidized gases, ammoxidation of methane, etc. The focus of the teachings of the '411 patent is the conversion of an oxygen-consuming gas, such as methane, to produce other useful gases, e.g. synthesis gas. Accordingly, in the case where methane is disposed on one side of the mixed ionic/electronic conducting membrane and air is disposed on the opposite side of the membrane, as the air contacts the membrane, the oxygen component of the air is reduced to oxygen ions which are transported through the membrane to the methane side of the membrane where the oxygen ions react with the methane to produce synthesis gas comprising primarily hydrogen and carbon monoxide or to produce olefins, depending upon the reaction conditions. In accordance with another embodiment, the oxygen-containing gas on one side of the membrane is a gas containing steam, i.e. H2O gas. The H2O contacts the membrane resulting in reduction of the oxygen in the H2O to oxygen ions which are transported across the membrane to the opposite side where they react with methane or natural gas to produce a synthesis gas (primarily H2 and CO) and the H2O on the first side of the membrane is reduced to hydrogen, which may be recovered and used for any number of purposes.